The present invention relates to a non-magnetic undercoating layer for a magnetic recording medium, a magnetic recording medium having a non-magnetic undercoating layer, and non-magnetic particles for a non-magnetic undercoating layer. More particularly, the present invention relates to a non-magnetic undercoating layer for a magnetic recording medium which has an excellent surface smoothness and a high strength, or which has an excellent surface smoothness, a high strength and protection property against oscillation (vibration proofness), a magnetic recording medium having such a non-magnetic undercoating layer, and non-magnetic particles for such a non-magnetic undercoating layer.
With a development of miniaturized and lightweight video or audio magnetic recording and reproducing apparatuses for long-time recording, magnetic recording media such as a magnetic tape and magnetic disk have been strongly required to have a higher performance, namely, a higher recording density and to reduce the noise level.
Especially, video tapes are required to have a higher picture quality, and the frequencies of carrier signals recorded in recent video tapes are higher than those recorded in conventional video tapes. In other words, the signals in the shortwave region have come to be used and, as a result, the magnetization depth from the surface of a magnetic tape has come to be remarkably small (shallow).
With respect to signals having a short wavelength, effort has also been made to improve the high output characteristics, especially, the S/N ratio, as described in the following literature. For example, Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, published by Kabushiki Kaisha Sogo Gijutsu Center (1982) says on page 74, " . . . In the recording and reproducing characteristics, technical problems in designing a magnetic coating layer so as to produce a high recording density by achieving various requirements in electromagnetic conversion property such as a reduction in the noise level, improvement of the S/N ratio, the sensitivity and the frequency characteristics, and a reduction in the output fluctuation are (1) to improve the uniform dispersibility of magnetic particles and the magnetic orientation, (2) to increase the packing ratio-of magnetic particles in a coating film, and (3) to provide a coating film with an excellent surface smoothness and a uniform thickness. . . . ", and on page 312, " . . . the conditions for high-density recording in a coating type tape are that the noise level is low with respect to signals having a short wavelength and that the high output characteristics are maintained. To satisfy these conditions, it is necessary that the tape has large coercive force (Hc) and residual magnetization (Br), . . . the coating film has a smaller thickness. . . . ".
Development of a thinner film for a magnetic recording some problems.
Firstly, it is necessary to make a magnetic recording layer smooth and to eliminate the non-uniformity of thickness. As well known, in order to obtain a smooth magnetic recording layer having a uniform thickness, the surface of the base film must also be smooth. This fact is described on pages 180 and 181 of Materials for Synthetic Technology-Causes of Abrasion of Magnetic Tape and head Running System and Measures for Solving the Problem (hereinunder referred to as "Materials for Technology" (1987), published by the publishing department of Technology Information Center, " . . . the surface roughness of a hardened magnetic layer depends on the surface roughness of the base film (back surface roughness) so largely as to be approximately proportional, . . . since the magnetic layer is formed on the surface of the base film, the more smooth the surface of the base film is, the more uniform and larger head output is obtained and the more the S/N ratio is improved."
Secondly, a problem in the strength of a base film has been caused with a tendency of the decreasdc in the thickness of a base film in response to the demand for a thinner magnetic layer. This fact is described, for example, on page 77 of the above-described Development of Magnetic Materials and Technique for High Dispersion of Magnetic Powder, " . . . Higher recording density is a large problem assigned to the present magnetic tape. This is important in order to shorten the length of the tape so as to miniaturize a cassette and to enable long-time recording. For this purpose, it is necessary to reduce the thickness of a base film . . . . With the tendency of reduction in the film thickness, the stiffness of the tape also reduces to such an extent as to make smooth traveling in a recorder difficult. Therefore, improvement of the stiffness of a video tape both in the machine direction and in the transverse direction is now strongly demanded . . . . "
Thirdly, there is a problem of too large a light transmittance caused by ultra-fine magnetic particles and a thin magnetic layer. Travel of a magnetic recording medium such as a magnetic tape, especially, a video tape is stopped when the video deck detects a portion of the magnetic recording medium at which the light transmittance is large. If the light transmittance of the whole part of a magnetic recording layer is made large by the production of a thinner magnetic recording medium or the ultra-fine magnetic particles dispersed in the magnetic recording layer, it is difficult to detect the portion having a large light transmittance by a video deck. As a measure for reducing the light transmittance of the whole part of a magnetic recording layer, carbon black or the like is added to the magnetic recording layer. It is, therefore, essential to add carbon black or the like to a magnetic recording layer in the video tapes.
Also, when a magnetic tape runs, it comes into vibrating by contacting with a magnetic head. The vibration has a close relationship with the thickness of a magnetic recording layer, and the finer the magnetic particles become and the thinner the magnetic recording layer becomes, the larger the vibration of the magnetic tape is apt to become. In this case, the output envelope of the magnetic tape may lack uniformity, so that the electromagnetic conversion (signal recording) output becomes unstable. Therefore, a magnetic tape having a thin recording layer is also required almost never to vibrate when the magnetic tape runs (this property will hereinunder be referred to as "vibration proofness").
As examples of the related art regarding a magnetic recording medium composed of a non-magnetic substrate and at least one undercoating layer produced by dispersing non-magnetic particles in a binder, Japanese Patent Application Laid-Open (KOKAI) Nos. 63-187418 (1988), 4-167225 (1992) and 5-182177 (1993) will be cited.
Japanese Patent Application Laid. Open (KOKAI) No. 63-187418 (1988) proposes a magnetic recording medium comprising a non-magnetic substrate, at least one undercoating layer produced by dispersing non-magnetic particles in a binder, and a magnetic layer produced dispersing ferromagnetic particles in a binder, wherein the ferromagnetic particles are ferromagnetic iron oxide particles, ferromagnetic cobalt-modified iron oxide particles or ferromagnetic alloy particles, the average major axial diameter of the ferromagnetic particles measured through a transmission electron microscope is less than 0.30 .mu.m and the crystalline size thereof by X-ray diffraction is less than 300 .ANG.. According to the specification of Japanese Patent Application Laid-Open (KOKAI) NO. 53-187418 (1988), the non-magnetic particles used for the undercoating layer are carbon black, graphite, titanium oxide, barium sulfate, ZnS, MgCO.sub.3, ZnO, CaO, .gamma.-iron oxide, tungsten disulfite, molybdenum disulfite, boron nitride, MgO, SnO.sub.2, SiO.sub.2, Cr.sub.2 O.sub.3, .alpha.-Al.sub.2 O.sub.3, SiC, cerium oxide, corundum, synthetic diamond, .alpha.-iron oxide, garnet, quartzite, silicon nitride, silicon carbide, molybdenum carbide, boron carbide, tungsten carbide, titanium carbide, tripoli, diatomaceous, dolomite or the like.
Japanese Patent Application Laid-Open (KOKAI) No. 4-167225 (1992) proposes a magnetic recording medium produced by forming a magnetic layer on the surface of a non-magnetic substrate through an undercoating layer which contains acicular or spindle-shaped particles having an aspect ratio of more than 3.0 in a resin binder hardened when irradiated with an electromagnetic wave such as radioactive rays and ultraviolet rays.
Japanese Patent Application Laid-Open (KOKAI) No. 5-182177 (1993) proposes a magnetic recording medium comprising a non-magnetic substrate, a non-magnetic undercoating layer formed on the non-magnetic substrate by dispersing an inorganic powder in a binder, and an upper magnetic layer formed on the non-magnetic undercoating layer by dispersing a ferromagnetic powder in a binder while the non-magnetic undercoating layer is wet, wherein the thickness of the upper magnetic layer in the dried state is not more than 1.0 .mu.m, and the non-magnetic undercoating layer contains a non-magnetic inorganic powder coated with an inorganic oxide. Japanese Patent Application Laid-Open (KOKAI) No. 5-182177 (1993) also proposes a magnetic recording medium comprising a non-magnetic substrate, a non-magnetic undercoating layer formed on the non-magnetic substrate by dispersing an inorganic powder in a binder, and an upper magnetic layer formed on the non-magnetic undercoating layer by dispersing a ferromagnetic powder in a binder while the non-magnetic undercoating layer is wet, wherein the thickness of the upper magnetic layer in the dried state is not more than 1.0 .mu.m, the non-magnetic undercoating layer contains a non-magnetic inorganic powder coated with at least one inorganic oxide selected from the group consisting of Al.sub.2 O.sub.3, SiO.sub.2 and ZrO.sub.2, the amounts of Al.sub.2 O.sub.3, SiO.sub.2 and ZrO.sub.2 being 1 to 21 wt %, 0.04 to 20 wt % and 0.05 to 15 wt %, respectively, based on the total weight of the non-magnetic inorganic powder, the amount of the non-magnetic inorganic powder is 51 to 99.8 wt % based on the whole inorganic powder contained in the non-magnetic undercoating layer, and the non-magnetic inorganic powder contains futile titanium dioxide as the main ingredient and 5 to 30 wt % of the inorganic oxide.
As described above, it is essential to add carbon black or the like to a magnetic recording layer in order to solve the problem of the light transmittance increasing by reducing the thickness of the magnetic recording layer.
However, addition of non-magnetic particles such as carbon black not only impairs the enhancement of the recording density but also reduces the magnetization depth from the surface of the magnetic tape. It is, therefore, unfavorable to add the non-magnetic particles to a magnetic recording layer.
With the development of a thinner film for a magnetic recording layer and a base film, a magnetic tape which has an excellent surface smoothness, a high strength and vibration proofness is now in the strongest demand, but no magnetic tape which satisfies all of these demands has been produced yet.
Japanese Patent Application Laid-Open (KOKAI) No. 63-187418 (1988) states that by providing at least one undercoating layer produced by dispersing non-magnetic particles in a binder on a non-magnetic substrate by the method disclosed therein, the light transmittance is improved and the problem of deterioration of the surface properties and signal recording property is solved.
As seen from the above, many materials such as hematite, barium salfate, titanium oxide, etc. are described in the specification of Japanese Patent Application Laid-Open (KOKAI) No. 63-187418 as non-magnetic particles and granular, acicular or spindle-shaped particles having particle diameter in a very wide range are said to be usable.
However, as a result of investigation of the above-described non-magnetic particles by the present inventors, it has been found that the surface smoothness and the strength are unsatisfactory not only when granular non-magnetic particles are used but also when acicular or spindle-shaped non-magnetic particles are used.
In the case of using acicular or spindle-shaped FeOOH particles which are described as the acicular or .alpha.-spindle-shaped particles in Japanese Patent Application Laid-Open (KOKAI) No. 4-167225, since much crystal water is contained in the surfaces of acicular or spindle-shaped .alpha.-FeOOH particles, the conformity of the particles with a binder resin and a solvent is so poor that the desired dispersibility is not obtained.
In order to obtain a high surface smoothness in a non-magnetic undercoating layer, it is necessary to disperse non-magnetic particles in a binder resin to a very high degree. The dispersibility of coatings for a non-magnetic undercoating layer composed of non-magnetic particles, a binder resin, a solvent, etc. can be expressed by a molecular weight-dependent parameter a (hereinunder referred to merely as "parameter .alpha."), as described on pages 94 to 96 of Explication and Applied Technique of Dispersion and Agglomeration (1992), published by Kabushiki Kaisha Technosystem, " . . . Many natural and synthetic polymers are adsorbed onto the surface of colloidal particles and form a thick adsorption layer, which exerts a great influence on the stability of the dispersion system. The following relationship generally holds between the molecular weight (M) and the saturation adsorption (As) of a polymer: EQU As=K.sub.1 .multidot.M.sup..alpha.
wherein K.sub.1 is a constant characteristic of the system, and .alpha. is called a molecular weight-dependent parameter, which is also characteristic of the system, and changes between 0 and 1 depending upon the structure of the adsorption layer. . . . When .alpha.=1, the polymer is adsorbed at the end of a molecule. The saturation adsorption (As) is proportional to the molecular weight (M). In this system, since the polymer forms a forest of the deepest adsorption layers on the particle surfaces, a strong steric repulsion effect is produced and effectively contributes to the stability of the dispersion system . . . "
It has been found that when magnetic particles are obtained by coating the surface of an inorganic powder such as .alpha.-iron oxide with an inorganic oxide such as Al.sub.2 O.sub.3 and SiO.sub.2 as described in Japanese Patent Application Laid-Open (KOKAI) No. 5-182177 (1993), the parameters .alpha. is about 0.40 to 0.45, while the parameters .alpha. of the particles which are not subjected to such a surface coating treatment described in Japanese Patent Application Laid-Open (KOKAI) No. 5-182177 (1993) is about 0.28 to 0.34.
Accordingly, the present invention aims at providing a non-magnetic undercoating layer for a magnetic recording medium which has an excellent surface-smoothness and a high strength, and which enables a thin magnetic recording layer having a small light transmittance, an excellent surface smoothness and a uniform thickness to be formed thereon by using .alpha.-Fe.sub.2 O.sub.3 particles having an excellent dispersibility which is expressed by an parameter .alpha. of larger than 0.50.
Although many materials such as .gamma.-iron oxide, .alpha.-iron oxide, barium sulfate or the like are described in Japanese Patent Application Laid-Open (KOKAI) No. 63-187418 (1988) and the surface smoothness and the strength of a magnetic tape are improved, the vibration proofness thereof cannot be said to be satisfactory.
Accordingly, the present invention also aims an providing a non-magnetic undercoating layer for a magnetic recording medium which has an excellent surface smoothness, a high strength and a vibration proofness, and which enables a thin magnetic recording layer having an excellent surface smoothness and a uniform thickness to be formed thereon and a vibration-proof magnetic tape to be produced therefrom.
As a result of studies undertaken by the present inventors so as to achieve such aims, it has been found that a non-magnetic undercoating layer containing the following non-magnetic particles (1) and/or (2), has an excellent surface smoothness and a high strength, and that such the non-magnetic thin magnetic recording layer provided on the undercoating layer has a small light transmittance, an excellent surface smoothness and a uniform thickness.
(1) Coated acicular or spindle-shaped .alpha.-Fe.sub.2 O.sub.3 particles which are produced by adhering an oxide or a hydroxide containing Al, Si or both Al and Si to the surfaces of acicular or spindle-shaped .alpha.-Fe.sub.2 O.sub.3 particles, and in which the molecular weight-dependent parameter .alpha. represented by the following formula is not less than 0.5: EQU As=K.sub.1 .multidot.M.sup..alpha.
wherein M represents the number-average molecular weight of a binder resin, As represents the saturation adsorption of the binder resin, and K.sub.1 represents a constant used for measuring As and dependent on the binder resin and a solvent.
(2) BAO.4.5 Fe.sub.2 O.sub.3 particles having an average particle diameter of not more than 0.1 .mu.m.
On the basis of this finding, the present invention has been achieved.